The present invention relates to a semiconductor laser module for introducing a beam of light issuing from a semiconductor laser into a single mode optical fiber.
An optical communication system which is implemented with an optical fiber includes an optical signal transmitting section in which a coherent beam of light emanating from a semiconductor laser (laser diode: LD) is modulated by a predetermined electric signal and, then, introduced into an optical fiber. In an LD module of the kind directing an output beam of an LD into a single mode optical fiber, the beam from an LD is condensed by a lens while an end face of an optical fiber is positioned in the vicinity of an imaging plane of the lens.
A problem heretofore pointed out in relation to such an arrangement is that a part of the beam which is reflected by the end face of the optical fiber is apt to be returned into the LD to render the wavelength spectrum of the LD unstable due to interference.
One prior art implementation for the reduction of the return of reflection is an LD module having a transparent sheet glass which is fixed in place on an end face of an optical fiber to which a beam from an LD is incident, that end of the sheet glass which receives the laser beam being inclined relative to a plane which is perpendicular to an axis of the optical fiber (see a paper entitled "Characteristics of LD Module for Single Mode Fiber 400 Mb/s", (in Japanese) Papers of National Meeting of the Institute of Electronics and Communication Engineers of Japan, pp. 8-360, published March 5, Show a 57 (1982)). Another prior art implementation is an LD module in which that end face of an optical fiber which receives a beam from an LD is inclined itself relative to a plane which is perpendicular to an axis of the optical fiber. While both of such prior art LD modules allow the beam from the LD to be focused on the end face of the optical fiber, they bring about a decrease in the coupling efficiency of the LD and the optical fiber since the sheet glass or the fiber end itself is inclined as stated above. The sheet glass type scheme has another disadvantage that the beam from the LD reaches the optical fiber after being refracted by the inclined end of the sheet glass, further aggravating the coupling efficiency.